This invention relates to band-pass filters and more particularly to band-pass filters using an analog delay circuit in which the delay time is set by a clock signal.
Among conventional band-pass filters is the N-path filter whose center frequency can be set in accordance with a clock frequency. An N-path filter consists of a resistor and N capacitors, each of which, when connected to the resistor through a switch opened and closed by a clock, constitutes in combination with the resistor a lowpass filter or, when separated therefrom, acts as an analog voltage memory. Sequential scanning of these N capacitors arranged in parallel effectively results in memorization of a period of the input waveform at N points. Therefore, in order to reduce distortion due to the scanning or sampling, the number of samples taken in a period, i.e., N, should be increased but this requires a corresponding increase in number of components of the circuit. For details of such N-path filters, reference is made to the following literature:
1. Franks et al., "An Alternative Approach to the Realization of Network Transfer Functions: The N-Path Filter", The Bell System Technical Journal, September 1960, pp. 1,321 thru 1,350;
2. Harden, "Digital Filters with IC's Boost Q without Inductors", Electronics, July 24, 1967, pp. 91 thru 100; and
3. U.S. Pat. No. 3,783,413 issued on Jan. 1, 1974.